Pummeled by overfishing, the world's coral reef ecosystems "will not survive for more than a few decades" unless drastic action is taken to protect them, experts warn.

To forestall a disaster that could devastate marine life, expose populous coastlines to stormier waves and economically devastate a tourism-dependent nation like Australia, the United States and other nations should vastly expand the designated "no take" zones -- where fishing and other exploitation is banned -- in coral ecosystems, said one author of an article for Friday's issue of Science.

Historical evidence dating back thousands of years proves that overfishing, not recent coral diseases or other causes, is the main cause of the slow death of the world's coral ecosystems, marine paleontologist John Pandolfi of the Smithsonian's National Museum of Natural History and 11 other researchers say in the article.

"Overfishing seems to be the largest 'signal' that explains our data," Pandolfi said.

So far they've documented only one out-and-out extinction of a coral reef inhabitant -- the Caribbean monk seal. But over the centuries, many other coral reef denizens have declined to the point where they have "no ecological impact -- they're functionally 'gone,' like coral trout, snapper, many of the turtles, (and) the manatees," Pandolfi said.

Because coral ecosystem life forms are so interdependent, the continual loss of species "is like taking bricks out of a building, one by one. At a certain point the building is going to come crashing down," he added. "There are places like Jamaica where the percentage of live coral (as opposed to dead coral) is down to 5 percent."

As moviegoers who've seen "Finding Nemo" know, a coral reef "provides a lot of places for fish to live. Coral reefs occupy about 0.2 percent of the world's oceans, yet they contain 25 percent of the species diversity," Pandolfi said.

For fish, coral reefs are combination condos and restaurants. They're attractive to fish partly because they provide shelter from predators and all the food they can swallow. They also offer numerous idiosyncratic ecological "niches" for those oddball fish -- the loners and bohemians of the undersea world -- who prefer to, say, burrow into the sand beneath the coral rather than hobnob within the coral complex itself.

Ever since humans began fishing thousands of years ago, species that jam the undersea metropolises called coral ecosystems have been gradually disappearing -- the biggest species first, such as green turtles -- according to the researchers' analysis of historical and archaeological records.

They pored over documents such as Colonial-era records of fish catches from 14 coral reef ecosystems in the Atlantic and Pacific oceans and Red Sea, including Australia's Great Barrier Reef and coral reefs of the Caribbean.

One of the 12 co-authors, zoology Professor Karen A. Bjorndal of the University of Florida, and her colleagues found more than 400 documents, some of them going back to the British colonial era and earlier, that recorded fish catches over the centuries in the Bahamas alone. They learned that native Bahamians severely depleted the coral ecosystem's green turtles long before the Brits arrived.

"I used to think that green turtles were basically in pristine shape when Columbus arrived (in Bahama five centuries ago), and I don't think that anymore," Bjorndal said in a press release issued by the university.

Based on the historical records, overfishing should be targeted as the No. 1 cause of coral ecosystem decline, the scientists concluded.

As an analogy, "imagine if 90 percent of the redwoods disappeared in Northern California," Sala said.

One solution: No-take zones should be greatly expanded in the world's coral ecosystems, Pandolfi said in a phone interview. The U.S. government has already designated five percent of coral ecosystems under its control as no- take zones. But Pandolfi advocates boosting the percentage to as high as 50 percent.

Pandolfi cites a legal precedent: The state of California's recent move to greatly expand protection to marine ecosystems off its coast. During the last year, the California state Fish and Game Commission boosted to 11 percent the no-take share of the 1,500-square-mile Channel Islands National Marine Sanctuary off Santa Barbara and Ventura. The previous percentage was less than 1/10th of one percent, according to ocean environmental activists.

Besides threatening the food supply of much of the world, reef loss could imperil natural harbors that are sheltered by coral formation and could undermine tourism based on the appeal of vibrant coral life.

Failure to prevent continued coral reef deterioration could turn countries such as Australia -- which are dependent on tourism at attractions such as the Great Barrier Reef -- into "Third World countries," Pandolfi said.

Some anti-environmentalists might scoff, saying that humanity will continue to muddle through whatever happens to the coral reefs, Pandolfi acknowledged. He added: "If you want to live in a world where the ocean is mostly jellyfish and bacteria, there's nothing I can do about it."